The invention relates to numerically controlled machines used for machining workpieces, and particularly to methods for qualifying the accuracy of such machines to ensure that parts will be produced within acceptable tolerances. The invention relates more particularly to a process for qualifying the accuracy of a machining system having a numerically controlled machine and a flexible workpiece holding fixture, and for diagnosing sources of errors and correcting such sources of errors.
Numerically controlled machine tools are widely used for machining many types of parts. In the aircraft industry, gantry-type and post mill-type machines having multi-axis movement capabilities are used for machining wing and fuselage panels to form holes in which rivets, bolts, or similar fasteners will be installed for attaching various structures and components to the panels. The panels in many cases are quite large, and are held for machining by a flexible workpiece holding fixture whose configuration can be varied to match a given workpiece so that panels of various configurations can be fixtured. A machine tool, such as a five-axis tool that is translatable along three mutually orthogonal axes and rotatable about two orthogonal axes, is positioned opposite the holding fixture and is translated and rotated to position a tool held in a spindle of the machine in the proper locations for drilling holes through the workpiece or performing other machining operations.
In such a system, it will be appreciated that there are many degrees of freedom between the machine tool and the holding fixture. Accurate parts can be produced only if there is a high degree of confidence that the accuracy in positioning of the machine tool and the holding fixture are within acceptable limits. However, there are many potential sources of errors that can creep in, both within the machine tool and within the holding fixture. Potential sources of errors in the machine include mechanical misalignments of and between the linear ways of the machine along which the machine travels, and mechanical misalignments of and between the rotational axes of the machine. Additionally, where the workpiece holding fixture includes holding elements that can be variably positioned along one or more axes, the fixture may introduce further inaccuracies along and between such axes.
Traditional methods for checking and correcting positioning errors in such machine systems have relied heavily upon recalibrating and realigning the machine to original factory specifications when the errors become unacceptably large. This can involve the replacement of parts of the machine that can no longer deliver performance up to par with the original specifications. Many times, the errors in positioning are judged by inspecting the finished part quality and noting when the parts become out of tolerance. This is an inherently reactive rather than proactive process, and inevitably unacceptable parts will be produced at some point when the machine accuracy declines as a result of wear, shifts between parts of the machine, or other causes.
A further drawback to this traditional approach is that it may well be possible to produce parts within acceptable tolerances even though the machine does not meet original factory specifications. Accordingly, realigning the machine to original specifications may result in needless downtime and expense. In order to efficiently correct inaccuracies without rebuilding the machine to original specifications, however, the root causes of the errors must be known. In the traditional approach to machine accuracy qualification, errors are first noted by checking the finished part quality, but this yields little or no insight into what is causing the parts to be produced out of tolerance. The traditional approach therefore involves a long and cumbersome process of measuring the linearity and straightness of each axis of the machine, the squareness between each pair of orthogonal axes, the alignment of the rotation axes, and other parameters, and correcting any unacceptably large inaccuracy by realigning the axes and replacing parts as needed in order to reestablish the original factory specifications. In this process, the true root causes of errors may not ever be discovered; it is simply hoped that by realigning the machine to original specifications, the finished part quality will be restored to an acceptable level.
A further drawback to many prior machine accuracy qualification methods is that the ultimate error in production part accuracy is never linked mathematically to the various contributing factors in the machine and/or holding fixture for the part, and hence there is no systematic way to check the machine and fixture accuracy that will assure that parts will be produced within acceptable tolerances. Accordingly, it is generally necessary to inspect the finished parts to determine whether the machine system is performing acceptably. It would be desirable to provide a machine accuracy qualification method in which the production part accuracy is mathematically linked to the various potential sources of error in the machine and holding fixture, enabling a high degree of confidence in production part accuracy to be achieved without having to regularly inspect the parts. In short, many prior attempts to maintain a high level of confidence in the accuracy of machines and fixtures have failed because of a misunderstanding of what needs to be checked, because the time between accuracy checks was unacceptably long, and because methods for collecting, analyzing, and reporting measurement data were incomplete and the results difficult to interpret.
The present invention seeks to overcome the drawbacks of prior approaches noted above. In accordance with the present invention, a hierarchical process for checking machine and fixture accuracy is provided that leads to the identification of the likely root causes of errors so that, if necessary, physical intervention can be taken to correct them. In many cases, however, the errors can be corrected without physical intervention, by making corrections within software. Machine and fixture positioning errors are related through a mathematical model to the statistical total error in the position of the machine tool (e.g., a drill bit for drilling holes), and maximum allowable amounts are assigned for each of the individual contributing factors to the total error. The math model in accordance with the invention allows all of the errors or tolerances of the machine and holding fixture to be distributed in a realistic manner in order to keep the resulting accuracy in production parts within acceptable limits. Periodic machine and fixture accuracy checks are made to assess whether each of the contributing causes of the statistical total machine tool position error is within its assigned tolerance band. The invention provides a systematic accuracy qualification process for making the periodic checks in such a manner that the root causes of the machine and fixture errors can readily be traced and corrected. The invention thereby seeks to allow the highest possible levels of hardware inaccuracies that still enable parts to be produced within acceptable tolerances. In this manner, the frequency of physical intervention is minimized, and when physical intervention is necessary, the likely root cause of error is identified so that extensive downtime to identify and correct the cause is unnecessary.
The process of the invention also enables the accuracy of the machine and fixture to be determined without requiring any foundation-based reference system or any other reference system external to the machine. Instead, a master-slave relationship is defined between the machine and the holding fixture, and the accuracy of the machine and fixture are checked within a master frame of reference that is relative to the axes along which the machine moves. Accordingly, it is not necessary in most cases to calibrate the machine to ground and gravity, as required in many prior art processes, in order to assure that production parts will be produced within tolerance.
To these ends, the process in accordance with at least some embodiments of the invention involves making regular periodic checks of the machine positioning accuracy and the relationship between the machine and the workpiece holding fixture using a measurement probe that is mounted in the tool-holding spindle of the machine where the machine tool (e.g., a drill bit or the like) would ordinarily be held. The checks are performed in a particular order such that in each check, errors can be attributed to a cause or multiple causes that are substantially independent of any other inaccuracies that have not already been identified and corrected in previous steps of the process. The process is suitable for use with any multi-axis machine tool. Embodiments of the invention described herein focus particularly on a machine of the type that travels along linear ways defining translation axes of the machine; typically, there are at least two orthogonal translation axes, and often there are three mutually orthogonal axes along which the machine can be moved to position the tool at any point in a three-dimensional volume. For example, the machine may include a gantry, post, or other prime mover that travels along ways on a floor along an X-axis direction, and a ram of the machine may travel vertically along the prime mover along a Y-axis and forward and backward on the prime mover along a Z-axis. The machine frequently will be further capable of rotating the tool about one or more axes so that the orientation of the tool axis can be varied. For example, the spindle may be mounted on a wrist that is mounted on the ram, the wrist having a head that rotates on the ram about one rotational axis and a body rotatably mounted in the head about another rotational axis. Based on the present disclosure it will be recognized by those of ordinary skill in the art, however, that the methods of the present invention can be applied to any multi-axis numerically controlled machine.
Thus, in one embodiment, the measuring probe is mounted in the machine""s spindle, and initially the alignment of the probe with the rotation axis of the head of the machine wrist is checked. More particularly, the accuracy of alignment of the probe relative to the rotation axis of the head is measured by rotating the head about its axis and moving the machine to cause the probe to contact a fixed monument or reference surface. If the probe measures a different position for the monument in two different rotational positions of the head, this is an indication of probe misalignment or other abnormality. The likely causes of the shift in measured position of the monument are traced and corrected.
The accuracy with which the machine can be repeatably positioned along each of its translation axes is checked by moving the machine independently along each axis to cause the probe to contact the monument, and detecting inaccuracies based on the positions of the monument measured by the probe. If this check fails, then the probe length may be different from what it is supposed to be, or the monument may have shifted, or the machine may have shifted. Each potential cause of errors in rotational and translational positioning accuracy and axis alignment is investigated, and corrected if necessary, before proceeding further with the accuracy qualification process, or before proceeding with the machining of parts.
The checks with the probe against the monument are useful for detecting certain probe-caused errors and machine-caused errors that may result in parts being produced out of tolerance. However, there are many other potential sources of errors that the monument checks cannot detect. For example, the monument checks do not provide any information as to potential errors that may be introduced in the workpiece holding fixture and the relative positioning of the fixture and machine. Additionally, the monument checks are not designed to check linearity and orthogonality of the machine""s axes over the entire working envelope of the machine, nor are they designed to check accuracy of the translational and rotational positioning of the machine over the entire ranges of machine movements that are employed during production.
Accordingly, after the monument checks have been satisfactorily completed, and any inaccuracies detected therein have been traced and corrected, the measurement probe is used to probe the workpiece holding fixture. In one embodiment, the fixture includes a plurality of holding members that are movable along at least one axis so that the fixture can be tailored to the configuration of the workpiece being machined. More preferably, the fixture""s holding members can be moved independently along two or three mutually orthogonal axes. The fixture is numerically controlled by a control unit of the fixture, which makes its own internal determination of where the holding members are positioned, for instance according to feedback position signals from actuators that move the holding members and respond to control signals telling the actuators where to position the holding members. It will be appreciated that if the holding members are not positioned where the machine xe2x80x9cthinksxe2x80x9d they are positioned, then inaccuracies will be introduced in finished parts. Thus, the fixture is configured with the holding members placed in predetermined positions, and the machine is moved to cause the probe to contact the holding members along each axis along which the holding members are movable. The probe-measured positions of the holding members are compared to desired positions, and delta values representing the discrepancies between actual and desired positions are determined along each axis for each holding member. The delta values are then accounted for in the software of the holding fixture""s control unit so as to xe2x80x9czero outxe2x80x9d the discrepancies. Ideally, the holding fixture can then be placed in the probed configuration and the holding members will be where they are supposed to be.
To check the holding fixture over the entire working envelope in which it may be used, a post-calibration check preferably is done by placing the fixture in various other configurations with the holding members in positions different from those used in the initial probe check. The actual positions of the holding members as determined by the machine and probe are compared with desired (i.e., the commanded) positions, and delta values are again calculated for each configuration. If the delta values in any configuration for any of the holding members exceed a threshold limit, then software corrections of the holding fixture are not adequate to correct the problem, and physical intervention is undertaken to bring the fixture back into acceptable accuracy so that it can pass the probe checks.
It is advantageous to perform the probe checks of the holding fixture on a regular periodic basis, but satisfactory control of the machine-fixture relationship may be maintained even though the probe checks of the fixture are performed less frequently than the monument checks of the machine accuracy. For example, the machine monument checks may be performed on a daily basis prior to the start of or between production operations, whereas the probe checks of the fixture may be performed weekly. Of course, these periods are merely exemplary.
As noted above, the order in which the various accuracy checks are performed forms a part of at least some embodiments of the invention. It is preferable to perform the checks in an order that rules out possible causes of inaccuracies in a logical order. For instance, a check of the holding fixture with the probe should not be done until it is known that the machine positioning accuracy is acceptable. Accordingly, a method for qualifying the machine accuracy in one embodiment includes the steps of:
(1) mounting a contact measuring probe in a spindle of the machine where a machine tool would ordinarily be mounted for machining, and checking positioning accuracy of the machine by moving the machine to cause the probe to contact a fixed monument in a known position such that the probe measures a position of the monument, and proceeding to the next step only if the probe-measured position of the monument is within a predetermined tolerance of the known position;
(2) subsequently checking positioning accuracy of the holding fixture by moving the machine to cause the probe to contact each of a plurality of holding members of the fixture that have been placed in positions determined within a numerical control unit of the holding fixture that controls positioning of the holding members, such that the probe measures a position of each holding member; and
(3) proceeding to machining operations only if the probe-measured position of each holding member that was checked is within a predetermined tolerance of the position determined within the numerical control unit of the holding fixture.
In a further embodiment, the length and alignment of the probe and the accuracy and alignment of the machine""s rotational axes are first qualified by moving the machine to probe the fixed monument, then the machine accuracy along its translational axes is checked using the probe against the monument. Once these checks have passed, the probe check of the holding fixture is performed.
In a particular embodiment of the invention described herein, the holding fixture has a plurality of parallel, spaced-apart columns each of which supports a plurality of extendable and retractable pogos with vacuum assemblies for suctioning a workpiece onto the holding fixture and securing it for machining. The pogos are translatable along the column on which they are mounted, in a direction nominally parallel to a Y axis of the machine. The columns are translatable in a direction nominally parallel to an X axis of the machine. The pogos extend and retract along a direction nominally parallel to a Z axis of the machine. The machine translates in the X direction along ways on a floor. A ram of the machine translates along the Y and Z axes on ways mounted on the machine. A wrist is rotatably mounted on the ram. A head of the wrist rotates on the ram about a C axis that is nominally parallel to the Z axis. A spindle-holding body of the wrist is rotatably mounted on the head so as to rotate about an A axis that is nominally perpendicular to the C axis. The spindle of the machine is mounted on the body. The spindle is translatable along a W axis toward and away from the A axis.
The measuring probe is mounted in the spindle such that the lengthwise axis of the probe is aligned along the W axis. The length of the probe from the A axis out to a tip of the probe is set to a predetermined length. The length and alignment of the probe are checked by moving the machine to cause the probe to contact a fixed monument mounted on the floor proximate the machine. Preferably, a first step involves placing the spindle-holding body of the machine in a home position in which the W axis is supposed to be colinear with the C rotation axis of the wrist. With all other axes of the machine fixed, the machine is moved along the X direction to cause the probe to touch two diametrically opposite points on the inner edge of a circular hole formed in a fixed monument. The plane of the circular edge of the hole lies in the nominal XY plane of the machine; thus, the axis of the probe is nominally parallel to the axis of the hole. The machine is then moved to position the probe at the midpoint between the two X points and is moved along the Y direction to cause the probe to touch two diametrically opposite points on the circular edge. From these X and Y points, the X and Y coordinates of the center of the circular hole are calculated. Next, the wrist of the machine is rotated 180 degrees about the C axis and X and Y points of the edge of the hole are measured with the probe. If the X and Y coordinates of the center of the hole calculated after the rotation about the C axis are not within predetermined tolerances of the coordinates calculated before the rotation, then it means that there is possible misalignment of the probe axis relative to the C axis. Various causes may be responsible for the difference in calculated centers of the hole, including bent microswitches in the probe mechanism, bending of the shaft of the probe, inaccuracy in the home position of the spindle-holding body about the A axis, and/or nonorthogonality of the A axis relative to the C axis. Each potential cause is investigated, and corrected if necessary, before proceeding to further steps of the process.
The length of the probe from the A axis to the probe tip is checked by positioning the wrist such that the probe extends parallel to one of the translational axes of the machine, such as the X axis, and moving the machine along this axis to cause the probe tip to contact one side of a fixed monument, and the position of the side of the monument is measured. The spindle-holding body is then rotated 180 degrees such that the probe extends in an opposite direction but still parallel with the translational axis of the machine, and the machine is moved along this axis to cause the probe tip to contact and measure the position of an opposite side of the monument. The distance between the opposite sides of the monument in the direction of the translational axis is known. Based on this known distance and the measured positions of the opposite sides of the monument, the length of the probe from the A axis to the tip is calculated. If the calculated length is not within a predetermined tolerance of a desired value, the possible causes of the discrepancy are investigated and corrected.
Once the monument checks have assured that the probe length is correct and the orthogonality of the A axis relative to the C axis is acceptable, the accuracy of the machine""s translational positioning along the X, Y, and Z axes is checked by moving the machine along each axis, one at a time, so as to probe a fixed monument having surfaces whose positions along each axis are presumed to be known. The monument advantageously can be the same monument used in the previous checks; more specifically, the accuracy of machine positioning along the X and Y axes is checked by probing the inner edge of the circular hole in the monument. X and Y position coordinates are measured for at least three points spaced apart about the edge of the hole, and from these points the X and Y coordinates of the center of the hole are calculated. If the calculated center is not within a predetermined tolerance of a previously taught center, the possible causes are investigated and corrected. Similarly, the machine accuracy along the Z axis is checked by moving the machine in the Z direction to probe a surface of the monument, and comparing the measured position of the surface with a known position, and possible causes of any unacceptably large discrepancy are investigated and corrected. The primary causes of discrepancies in the X, Y, and Z positions measured for the monument include a shift of the machine along its translational axes, or a shift of the monument.
After the machine translational accuracy has been checked with the probe, preferably the accuracy of rotational positioning of the wrist about the C axis is checked. The wrist is rotated about the C axis to a position in which the probe should nominally extend parallel to the X axis. The machine is moved along the Y axis to cause the probe to touch a surface of a fixed monument, and the Y position of this surface is measured by the probe. The wrist is then rotated 180 degrees about the C axis, and the machine is again moved to cause the probe to measure the Y position of the same surface. If the two measured Y positions do not agree with each other within a predetermined tolerance, the possible causes of the discrepancy are investigated and corrected. The primary cause is inaccuracy in rotational positioning of the wrist about the C axis; that is, the rotation was less than or more than 180 degrees by some amount.
Preferably, the accuracy check of the holding fixture is performed only after all of the monument checks have been successfully completed. In a pre-calibration check of the fixture, the control unit of the fixture is caused to extend or retract certain ones of the pogos to a predetermined Z position, for instance such that all of the selected pogos are at Z=12 inches. These pogos are spaced apart in the X and Y directions preferably over the entire working envelope of the machine. The movable columns on which these pogos are mounted are placed in predetermined X positions, and the pogos are placed in predetermined Y positions along each column. The machine is then moved along each X, Y, and Z axis to cause the probe to contact surfaces of the vacuum assemblies that are mounted on the ends of the pogos. The X, Y, and Z positions of these surfaces are measured by the machine""s probe and are compared with the positions determined by the fixture""s control unit. Delta values between the probe-measured and fixture-determined X, Y, and Z positions are calculated, and are used in the fixture""s control unit as offset values to xe2x80x9czero outxe2x80x9d the discrepancies.
A post-calibration check can be performed after the delta values have been applied in the fixture""s control unit. Preferably, pogos are placed in a plurality of different X, Y, and Z positions spaced throughout a working volume of the machine and are probed by the machine to determine X, Y, and Z delta values. If the delta values exceed predetermined tolerances, then zeroing out the discrepancies within the fixture""s control unit is not adequate to correct the fixture positioning inaccuracies, and physical intervention is needed to correct the problem.
It will be appreciated that the probe checks alone are not adequate to detect and diagnose certain sources of errors in machine accuracy. The monument checks are typically made with the machine moving over only a relatively small volume of space, whereas in production the machine must move over a much larger volume of space. The monument checks thus are not designed to detect errors in, for example, straightness of the machine ways or orthogonality of the translation axes. Accordingly, the process in accordance with preferred embodiments of the invention includes a further machine accuracy check to determine the overall global positioning accuracy of the machine along its translation axes and about its rotation axes. The global positioning check advantageously employs a three-dimensional position-measuring device, desirably a laser tracker or other laser measuring instrument, that is capable of measuring the three-dimensional coordinates of a target mounted on the machine adjacent the spindle where a tool would ordinarily be held. Preferably, the measuring instrument is first used to create a master frame of reference that is relative to the ways of the machine. This is accomplished by moving the machine along only one axis, for example the X-axis defined by ways that extend along the floor, and using the measuring instrument to measure the positions of the target at each of a plurality of points spaced along the X-axis direction. The same procedure is performed along the Y-axis, which is nominally perpendicular to the X-axis. A linear best fit of the points measured by the measuring instrument is performed to determine a mutually orthogonal set of coordinate axes defining a master frame of reference based on the machine""s own translation axes. This best-fit process distributes errors in linearity and orthogonality of the machine""s translation axes over the working envelope of the machine. By creating the master frame of reference relative to the bit ways of the machine, the invention eliminates the need for establishing a foundation-based reference system or any other reference system external to the machine.
Once the master frame of reference is determined, the machine accuracy is checked by moving the machine so as to position the spindle at each of a plurality of locations that are preferably spaced apart along each translation axis throughout the working envelope of the machine, measuring the coordinates of the target with the measuring instrument for each location, and determining errors in positioning of the machine by comparing the measured coordinates with machine-determined coordinates determined within the numerical control unit of the machine. If any discrepancy between the position measured by the measuring instrument and the machine-determined position exceeds a threshold limit (or, alternatively, if a statistical aggregation of all of the errors exceeds a threshold limit), then physical intervention is employed to correct the machine so that it can pass the global positioning accuracy check. Advantageously, the machine accuracy is checked only within the working envelope (i.e., the 3-D space within which the machine is required to move during machining operations), rather than checking the machine accuracy over the entire range of movement capability of the machine. This minimizes the time required to collect the accuracy data.
A similar positioning accuracy check is performed for the A and C rotation axes and the W spindle axis of the machine, by fixing the translation axes and placing the machine in various positions through movements about the A and C axes and along the W axis. The positions of the target are measured by the measuring instrument and are compared to the positions determined within the machine. Differences between the measured positions of the target and the machine-determined positions are compared with threshold limit values to determine whether physical intervention is needed to bring the machine within acceptable accuracy.
The machine accuracy positioning checks can also include dynamic accuracy checks wherein the machine is commanded to move along a predetermined curve (e.g., a circle) in each of the XY, YZ, and XZ planes, and the position of the target is measured at each of a plurality of points along the curve. Discrepancies between the measured positions and the desired positions can be due to various potential causes including lack of orthogonality between the axes of the machine or inaccuracy in translational positioning along the machine axes. The dynamic checks allow data from perpendicular planes to be directly related in straightness, squareness, and accuracy.
The global positioning accuracy checks using the measuring instrument are preferably performed on a regular periodic basis, but may be less frequent than the probe checks of the fixture. For instance, the global positioning accuracy check advantageously can be performed on a monthly basis.